The present invention relates to identification systems and methods, and more particularly to marking systems wherein identification marks are applied on preselected objects in such manner that the marks are not visible when viewed by the unaided eye but may only be observed under controlled conditions.
In the past, certain materials, which are often referred to as "invisible inks", have been used for a variety of purposes, such as the marking of checks, bank notes, bonds and other negotiable instruments so that any fraudulent alterations of the instruments may be detected. Invisible inks have also been used to mark laundry, to identify contents of containers and to authenticate identification badges. Although such marking systems have been satisfactory for most of the uses mentioned, there has developed a need for a secret marking system having a greater degree of security against detection. In all known instances wherein such invisible inks have been used in the past, the inks have been made of a material which has the characteristic of fluorescing when exposed to intense ultraviolet. Since the ultraviolet light that is available in daylight is not of sufficient intensity to cause fluorescence, these materials appear to be transparent and invisible in daylight. However, when exposed to intense ultraviolet radiation, these materials fluoresce and emit radiation in the visible spectrum which then is observable with the unaided eye.
Marking materials which fluoresce are not suitable for clandestine operations because any material which radiates in the visible spectrum when illuminated by ultraviolet could be noticed by the casual observer. Another reason that fluorescent type marking materials are not suitable for most military applications is that the necessarily high illumination intensity requirements make distant illumination and viewing impractical.
There are many instances when it is imperative that the presence of the mark be concealled from everyone except the person or persons who supplied the marks. In military operations, for example, it may be desirable to place registration plaques on all non-military boats, operating in a particular geographic area and then superimpose the invisible marking system of this invention on certain of those plaques to identify only those boats. With the identification system of this invention, all boats would bear registration plaques which appeared to be identical to the plaques on other boats but only the suspect boats would have the invisible secret mark superimposed on the plaque.